making art with air

this is research about my ideal public art project – some sort of large-scale lens that would let you see the air.

flow visualization

Shadowgram of the turbulent plume of hot air rising from a home-barbecue gas grill. Photograph by Gary S. Settles, Penn State University.

In experimental fluid dynamics, flows are visualized by three methods:

  • Surface flow visualization: This reveals the flow streamlines in the limit as a solid surface is approached. Colored oil applied to the surface of a wind tunnel model provides one example (the oil responds to the surface shear stress and forms a pattern).
  • Particle tracer methods: Particles, such as smoke, can be added to a flow to trace the fluid motion. We can illuminate the particles with a sheet of laser light in order to visualize a slice of a complicated fluid flow pattern. Assuming that the particles faithfully follow the streamlines of the flow, we can not only visualize the flow but also measure its velocity using the particle image velocimetry or particle tracking velocimetry methods.
  • Optical methods: Some flows reveal their patterns by way of changes in their optical refractive index. These are visualized by optical methods known as the shadowgraph, schlieren photography, and interferometry. More directly, dyes can be added to (usually liquid) flows to measure concentrations; typically employing the light attenuation or laser-induced fluorescence techniques.

Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function. Shadowgraph is a type of flow visualisation.

In principle, we cannot directly see a difference in temperature, a different gas, or a shock wave in the transparent air. However, all these disturbances refract light rays, so they can cast shadows. The plume of hot air rising from a fire, for example, can be seen by way of its shadow cast upon a nearby surface by the uniform sunlight.

The synthetic schlieren method is a technique similar to schlieren photography which makes use of digital photography and image processing rather than optics to visualize the density variations of a fluid.

Schlieren and shadowgraph techniques are basic and valuable tools in various scientific and engineering disciplines. They allow us to see the invisible: the optical inhomogeneities in transparent media like air, water, and glass that otherwise cause only ghostly distortions of our normal vision. These techniques are discussed briefly in many books and papers, but there is no up-to-date complete treatment of the subject before now. The book is intended as a practical guide for those who want to use these methods, as well as a resource for a broad range of disciplines where scientific visualization is important. The book was written by Professor Settles and was published by Springer in 2001. The following links are provided for you to conveniently purchase directly from Springer.

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